Chk-1 is a serine/threonine kinase involved in the induction of cell cycle checkpoints in response to DNA damage and replicative stress [Clin. Can. Res. 2007; 13(7)]. Cell cycle checkpoints are regulatory pathways that control the order and timing of cell cycle transitions. Most cancer cells have impaired G1 checkpoint activation due to a defective p53 tumor suppressor protein. Hahn et al., “Rules for making human tumor cells” N. Engl. J. Med. 2002; 347: 1593-603 and Hollstein et al., “p53 mutations in human cancers” Science 1991; 253: 49-53) have reported that tumours are associated with mutations in the p53 gene, a tumour suppressor gene found in about 50% of all human cancers.
Chk-1 inhibition abrogates the intra S and G2/M checkpoints and has been shown to selectively sensitise tumour cells to well known DNA damaging agents. Examples of DNA damaging agents where this sensitising effect has been demonstrated include Gemcitabine, Pemetrexed, Cytarabine, Irinotecan, Camptothecin, Cisplatin, Carboplatin [Clin. Cancer Res. 2010, 16, 376], Temozolomide [Journal of Neurosurgery 2004, 100, 1060], Doxorubicin [Bioorg. Med. Chem. Lett. 2006; 16:421-6], Paclitaxel [WO2010149394], Hydroxy urea [Nat. Cell. Biol. 2005 February; 7(2):195-20], the nitroimidazole hypoxia-targetted drug TH-302 (Meng et al., AACR, 2013 Abstract No. 2389) and ionising radiation [Clin. Cancer Res. 2010, 16, 2076]. See also the review article by McNeely, S., et al., “CHEK again: Revisiting the development of CHK1 inhibitors for cancer therapy, Pharmacology & Therapeutics (2014), http://dx.doi.orq/10.1016/j.pharmthera.2013.10.005.
Recently published data have also shown that Chk-1 inhibitors may act synergistically with PARP inhibitors [Cancer Res.; 66: (16)], Mek inhibitors [Blood. 2008 Sep. 15; 112(6): 2439-2449], Farnesyltransferase inhibitors [Blood. 2005 Feb. 15; 105(4):1706-16], Rapamycin [Mol. Cancer Ther. 2005 March; 4(3):457-70], Src inhibitors [Blood. 2011 Feb. 10; 117(6):1947-57] and WEE1 inhibitors (Chaudhuri et al., Haematologica, 2013.093187).
Resistance to chemotherapy and radiotherapy, a clinical problem for conventional therapy, has been associated with activation of the DNA damage response in which Chk-1 has been implicated (Chk-1 activation is associated with radioresistence in glioblastoma [Nature; 2006; 444(7):756-760] and the inhibition of Chk-1 sensitises lung cancer brain metastases to radiotherapy [Biochem. Biophys. Res. Commun. 2011 Mar. 4; 406(1):53-8]).
It is also envisaged that Chk-1 inhibitors, either as single agents or in combination, may be useful in treating tumour cells in which constitutive activation of DNA damage and checkpoint pathways drive genomic instability. This phenotype is associated with complex karyotypes in samples from patients with acute myeloid leukemia (AML) [Cancer Research 2009, 89, 8652]. In vitro antagonisation of the Chk-1 kinase with a small molecule inhibitor or by RNA interference strongly reduces the clonogenic properties of high-DNA damage level AML samples. In contrast Chk-1 inhibition has no effect on normal hematopoietic progenitors. Furthermore, recent studies have shown that the tumour microenvironment drives genetic instability [Nature; 2008; (8):180-192] and loss of Chk-1 sensitises cells to hypoxia/reoxygenation [Cell Cycle; 2010; 9(13):2502]. In neuroblastoma, a kinome RNA interference screen demonstrated that loss of Chk-1 inhibited the growth of eight neuroblastoma cell lines. Tumour cells deficient in Fanconi anemia DNA repair have shown sensitivity to Chk-1 inhibition [Molecular Cancer 2009, 8:24]. It has been shown that the Chk-1 specific inhibitor PF-00477736 inhibits the growth of thirty ovarian cancer cell lines [Bukczynska et al, 23rd Lorne Cancer Conference] and triple negative negative breast cancer cells [Cancer Science 2011, 102, 882]. Also, PF-00477736 has displayed selective single agent activity in a MYC oncogene driven murine spontaneous cancer model [Ferrao et al, Oncogene (15 Aug. 2011)]. Chk-1 inhibition, by either RNA interference or selective small molecule inhibitors, results in apoptosis of MYC-overexpressing cells both in vitro and in an in vivo mouse model of B-cell lymphoma [Hoglund et al., Clinical Cancer Research, Online First Sep. 20, 2011]. The latter data suggest that Chk-1 inhibitors would have utility for the treatment of MYC-driven malignancies such as B-cell lymphoma/leukemia, neuroblastoma and some breast and lung cancers. Ewing sarcoma cell lines have also been reported to be sensitive to Chk kinase inhibitors (McCalla et al., Kinase Targets in Ewing's Sarcoma Cell Lines using RNAi-based & Investigational Agents Screening Approaches, Molecular Targets 2013, Boston, USA).
It has also been reported that mutations that reduce the activity of DNA repair pathways can result in synthetically lethal interactions with Chk1 inhibition. For example, mutations that disrupt the RAD50 complex and ATM signaling increase responsiveness to Chk1 inhibition [Al-Ahmadie et al., Synthetic lethality in ATM-deficient RAD50-mutant tumors underlie outlier response to cancer therapy]. Likewise, deficiencies in the Fanconi anemia homologous DNA repair pathway lead to sensitivity to Chk1 inhibition [Chen et al., Chk1 inhibition asd a strategy for targeting fanconi anemia (FA) DNA repair pathway deficient tumors. Mol. Cancer 2009 8:24, Duan et al., Fanconi anemia repair pathway dysfunction, a potential therapeutic target in lung cancer. Frontiers in Oncology 2014 4:1]. Also, human cells that have loss of function in the Rad17 gene product are sensitive to Chk1 suppression [Shen et al., Synthetic lethal interaction between tumor suppressor RAD17 and Chk1 kinase in human cancer cells. 2014 SACNAS National Conference Abstract].
Various attempts have been made to develop inhibitors of Chk-1 kinase. For example, WO 03/10444 and WO 2005/072733 (both in the name of Millennium) disclose aryl/heteroaryl urea compounds as Chk-1 kinase inhibitors. US2005/215556 (Abbott) discloses macrocyclic ureas as kinase inhibitors. WO 02/070494, WO2006014359 and WO2006021002 (all in the name of Icos) disclose aryl and heteroaryl ureas as Chk-1 inhibitors. Our earlier applications WO/2011/141716 and WO/2013/072502 both disclose substituted pyrazinyl-phenyl ureas as Chk-1 kinase inhibitors. WO2005/009435 (Pfizer) and WO2010/077758 (Eli Lilly) disclose aminopyrazoles as Chk-1 kinase inhibitors.